The entry and exit of large molecules from the eukaryotic cell nucleus is tightly controlled by nuclear pore complexes (NPCs). Although small molecules can enter and exit the nucleus without regulation, macromolecules such as RNA, mRNA, ribosomal proteins, DNA polymerase, lamins, carbohydrates, signaling molecules, and lipids require association with karyopherins called importins to enter the nucleus and exportins to exit.
Nuclear pore complexes are large protein complexes that span the nuclear envelope, which is the double membrane surrounding the eukaryotic cell nucleus. The proteins that make up the nuclear pore complex are known as nucleoporins.
Nucleoporins are only required for the transport of large hydrophilic molecules above 40 kDa, as smaller molecules pass through nuclear pores via passive diffusion. For example, nucleoporins play an important role in the transport of mRNA from the nucleus to the cytoplasm after transcription. Depending on their function, certain nucleoporins are localized to a single side of the nuclear pore complex, either cytosolic or nucleoplasmic. Other nucleoporins may be found on both faces.
There are three distinct types of nucleoporins, each having a unique structure and function. These three types are structural nucleoporins, membrane nucleoporins, and FG-nucleoporins.
Structural nucleoporins form the ring portion of the NPC. They span the membrane of the nuclear envelope and are often referred to as the scaffolding of a nuclear pore. Structural nucleoporins come together to form Y-complexes that are composed of seven nucleoporins. Each nuclear pore contains sixteen Y-complexes for a total of 112 structural nucleoporins.
Membrane nucleoporins are localized to the curvature of a nuclear pore. These proteins are embedded within the nuclear membrane at the region where the inner and outer leaflets connect.
FG-nucleoporins are so named because they contain repeats of the amino acid residues phenylalanine (F) and glycine (G). FG repeats are small hydrophobic segments that break up long stretches of hydrophilic amino acids. These FG-repeat segments are found in long random-coil portions of the protein which stretch into the channel of nuclear pores and are believed to be primarily responsible for the selective exclusivity of nuclear pore complexes. These segments of FG-nucleoporins form a mass of chains which allow smaller molecules to diffuse through but exclude large hydrophilic macromolecules. These macromolecules are only able to cross a nuclear pore if they are associated with a transport molecule (karyopherin) that temporarily interacts with a nucleoporin's FG-repeat segments. FG-nucleoporins also contain a globular portion that serves as an anchor for attachment to the nuclear pore complex.
Karyopherins and their cargo are passed between FG-repeats until they diffuse down their concentration gradient and through the nuclear pore complex. The release of their cargo from karyopherins is driven by Ran, a G protein. Ran is small enough that it can diffuse through nuclear pores down its concentration gradient without interacting with nucleoporins. Ran binds to either GTP or GDP and has the ability to change a karyopherin's affinity for its cargo. Inside the nucleus, RanGTP causes an importin karyopherin to change conformation, allowing its cargo to be released. RanGTP can also bind to exportin karyopherins and pass through the nuclear pore. Once it has reached the cytosol, RanGTP can be hydrolyzed to RanGDP, allowing the exportin's cargo to be released.
Adapting artificially engineered protein polymers from consensus repeats of natural proteins is an attractive approach to mimic the unprecedented performance of natural materials. Tough silk-like polypeptides, thermoresponsive elastin-like polypeptides, and resilient and elastic resilin-like polypeptides have been synthesized to mimic the functions of natural materials. Important design principles have been developed for these artificial biopolymers that enable rational control over their thermodynamic, structural, and mechanical properties. The simplified repeat allows for a detailed understanding of sequence-structure-property relationships to be developed, and these tailor-made materials open up opportunities for applications such as drug delivery, tissue engineering, photonic films and smart responsive devices.
An additional natural material that has interesting engineering properties is the protein matrix which fills the nuclear pore complex (NPC) in the nuclear envelope and controls transport into the nucleus. It allows passage of less than 0.1% of all proteins while translocating over 1,000 molecules per pore per second. Ribbeck K et al., EMBO J 20: 1320 (2001); Yang W D et al., Proc Natl Acad Sci USA 101: 12887 (2004). The protein matrix is composed of nucleoporins, proteins containing Phe-Gly (FG) repeat sequences which contribute to specific binding of the nuclear transport receptors (NTRs) that facilitate transport of a specific subset of biological molecules into the matrix.
Individual nucleoporins can form hydrogels in vitro that recapitulate the enhanced permeability of selectively-labeled macromolecules into the gel, similar to the intact NPC, with varying degrees of passive diffusion of inert molecules. Labokha A A et al., EMBO J 32: 204 (2013); Jovanovic-Talisman T et al., Nature 457: 1023 (2009). This selectivity is rare in synthetic polymer hydrogels, making these natural materials an intriguing model for new filtration technologies. In spite of the advanced filtering function of natural nucleoporin hydrogels, until now a fundamental understanding of the sequence-structure-property relationships needed for materials engineering has been lacking, due to the complex sequence of the proteins and the inability to synthesize them recombinantly in high yields.